The Improved Knockdown fighter and mauler feat modifies the Knockdown feat action, specifically
You can dash your foe to the ground with a single blow. When you use Knockdown, instead of making a Strike followed by a Trip, you can attempt a single Strike. If you do and your Strike hits, instead of rolling a check for your Trip attempt, you automatically apply the critical success effect of a Trip. If you used a two-handed melee weapon for the Strike, you can use the weapon's damage die size instead of the regular die size for the damage from a critical Trip.
Notably, this doesn't say you make a Strike followed by a Trip any more – does this mean taking Improved Knockdown reduces the attack penalty "cost" of the knockdown action from two attacks to only one?
Best Answer
You Attempt a Single Strike
Knockdown has this line at the end about the multiple attack penalty:
Increasing your multiple attack penalty with each attack is the default rule for abilities with multiple attacks, but not increasing the penalty until the end would be the exception.
With Improved Knockdown you don't make two attacks, instead making a single Strike with the effects of a critically successful Trip attempt.
Only one attack is made so your multiple attack penalty only increases once.